The disease is any dangerous deviation from the normal state of the structure or function of the body, usually associated with certain signs and symptoms which are different from physical injury.

A sick creature often shows signs or symptoms that indicate its unusual condition. Thus, the general condition of the body must be understood to recognize the symptoms of the disease. However, a sharp distinction between disease and health is not always evident.

Disease research is called pathology. It involves determining the cause (etiology) of the disease, understanding the mechanisms for its development (pathogenesis), structural changes associated with the disease process (morphological changes), and the functional effects of those mutations. Proper diagnosis of the disease is necessary to determine the correct course of treatment.

In humans, diseases are often widely used to refer to any condition that causes pain, unemployment, depression, social problems, or death to a poor person, or similar problems to those who come in contact with a person.

In this broad sense, it sometimes includes injury, disability, disruption, disease, various symptoms, opposite behavior, and abnormal variation of structure and function, while in some cases and for other purposes these may be regarded as distinct categories. It can affect people not only physically, but also mentally, as being infected can change a person’s perception of health.

Deaths caused by diseases are called natural causes. There are four main types of diseases: infectious, degenerative, genetic  (including both genetic and non-genetic diseases), and autoimmune.

Diseases can be differentiated by other means, such as infectious and non-communicable diseases. The most deadly diseases in humans are coronary artery disease (obstruction of the bloodstream), followed by cerebral palsy and respiratory diseases.

In developed countries, the most common causes of illness are neuropsychiatric conditions, such as depression and anxiety.